The correlation between ABO/Rh blood groups and diabetes mellitus is still controversial. The aim of this study was to determine the relationship between ABO/Rhesus blood groups and diabetes in Turkish population. This cross-sectional study was conducted in Istanbul Medeniyet University Göztepe Education and Training Hospital's Diabetes Units. The study group was composed of 421 patients with type-1 diabetes, 484 patients with type-2 diabetes and 432 controls. Blood samples were collected and tested for ABO/Rhesus blood groups. Data was analyzed by SPSS version 17.0. A significant association was found between blood groups and diabetes mellitus. The frequency of AB blood group was significantly higher in type-1 diabetics; and A blood group was significantly higher in type-2 diabetics. Furthermore, Rh negativity were significantly more frequent in type-2 diabetics.